A new film based on the story of Austrian kidnap victim Natascha Kampusch shows her being repeatedly raped by captor who beat and starved her during the eight-and-a-half years that he kept her in a cellar beneath his house.

Kampusch was snatched on her way to school at the age of 10 by Wolfgang Priklopil and held in a windowless cell under his house near Vienna until she escaped in 2006, causing a sensation in Austria and abroad. Priklopil committed suicide.

Kampusch had always refused to respond to claims that she had had sex with Priklopil, but in a German television interview on her 25th birthday last week said she had decided to reveal the truth because it had leaked out from police files.

The film, 3,096 Days - based on Kampusch's autobiography of the same name - soberly portrays her captivity in a windowless cellar less than 6 square metres in area, often deprived at food for days at a time.

The emaciated Kampusch keeps a diary written on toilet paper concealed in a box. One entry reads: "At least 60 blows in the face. Ten to 15 nausea-inducing fist blows to the head. One strike with the fist with full weight to my right ear."

The movie shows occasional moments that approach tenderness, such as when Priklopil buys her a dress as a gift - but he immediately goes on to chide her for not knowing how to waltz with him.

A preview of the film starring British actress Antonia Campbell-Hughes as the teenaged Kampusch was shown to journalists on Monday ahead of the Vienna premiere which Kampusch and the film's director and stars are due to attend.

In an interview with Germany's Bild Zeitung last week, Kampusch said: "Yes, I did recognise myself, although the reality was even worse. But one can't really show that in the cinema, since it wasn't supposed to be a horror film."

The movie, made at the Constantin Film studios in Bavaria, Germany, also stars Amy Pidgeon as the 10-year-old Kampusch and Danish actor Thure Lindhardt as Priklopil.

The director is German-American Sherry Hormann, who made her English-language debut with the 2009 move Desert Flower, an adaptation of the autobiography of Somali-born model and anti-female circumcision activist Waris Dirie.

In the film, Kampusch is shown screaming in her cell, although she told Bild she never cried out in reality.

"My pain was a silent cry - although the feeling was exactly as in the film - like an electric shock," said Kampusch, whose daily routine now includes rising early, feeding the guppies in her aquarium, and watering her orchids.